Holography is an application of laser technology, best known for its ability to reproduce three dimensional images. Early holography was limited to using film to record intensity and phase information of light incident on the scene. More specifically, the principle of operation of film holograms is that the film records the interference pattern produced by two coherent beams of light, i.e., "recording beams". One recording beam is scattered from the scene being recorded and one recording beam is a reference beam. The interference patterns recorded on the film encode the scene's appearance from a range of viewpoints. Depending on the arrangement of the recording beams, and therefore the reconstructing and reconstructed beams, with respect to the film, the hologram may be a transmission-type or reflection-type hologram. For a transmission-type hologram, reconstructing the holographic image is accomplished by shining one of the recording beams, as a "reconstructing" beam, through the developed hologram. By diffraction, the recorded interference pattern redirects some of the light to form a replica of the other recording beam. This replica beam, the "reconstructed" beam, travels away from the hologram with the same variation in phase and intensity that the original beam had. Thus, for the viewer, the reconstructed wavefront is indistinguishable from the original wavefront, including the three dimensional aspects of the scene.
In recent years, computers have been used to generate interference patterns, thereby eliminating the need to capture the scene on film. The interference of the recording beams is simulated on the computer. The computed fringes may then be photographically recorded onto film.
Both photographic and computer-generated holograms require a very high resolution. The distance, L, from one interference fringe to the next depends on the wavelength, .lambda., of the light and the angle, .theta., between the directions of propagation of the wavefronts. Expressed mathematically, ##EQU1## For reasonable angles, this fringe spacing is about the size of a wavelength of visible light, which is about 2 micrometers. The resolution required to produce a useful hologram is at least 500 line pairs per millimeter. In the case of film, fine grain films are available to provide a resolution this fine or finer. In the case of computer-generated holograms, a pixel size of 1 micrometer is required to provide a resolution of 500 line pairs per millimeter.